The invention relates to a cannula for use, for example, in the administration and removal of fluids.
Conventional piercing cannulae have as a piercing part a more or less sharply inclined wedge-shaped ground portion of the tube end, which in a top view appears as an oval lumen opening in line with the bore of the tube, with the rear edge of the lumen opening forming a cutter. The edges of the lower zone of the piercing part are usually ground into a point.
In piercing, such a tip on the piercing part results in the fact that the sharp rear cutter of the lumen opening cuts a plug of this material necessarily forced into it from the surface to be penetrated, consequently enlarging the pierced hole.
On the one hand, this is disadvantageous with tissue piercing because of the traumatization, and, on the other hand, the plug-cutting effect proves to be extremely undesirable when such cannulae are used to pierce implantable catheter ports. Such systems consist as a rule of an implanted capsule which has a hollow bore for removal or administration of a fluid such as blood, blood components, or a medicament. The capsule is connected with a cathether which opens into a vessel or other site of action. The wall of the capsule toward the skin of the patient consists of a pierceable elastomer membrane which is penetrated by the cannula through the skin.
The piercing of a cannula through the membrane of the capsule causes breaks in the seal because of the fact that the cutter at the rear edge of the lumen opening cuts out a plug of elastomer material, resulting in holes which no longer close automatically due to the elastic force of the material. This leads to the circumstance that the implanted capsule already has leaks after a few insertions. Furthermore, elastomer plugs can be carried by the flow into the patient or block the catheter.
Because of these problems, to date so-called Huber cannulae have been used to pierce such administrative arrangements. In a generic cannula, known from U.S. Pat. No. 2,748,769, the front end of the cannula tube is bent to the side at an angle such that the wedge-shaped ground end of the tube runs substantially parallel to the longitudinal axis of the tube, whereby plug cutting of membrane material is supposed to be avoided. In this embodiment, the lumen opening is significantly outside the line of the tube. This circumstance can lead to damage to the membrane even with proper, perpendicular insertion of the cannula, since the angled front section of the cannula tube penetrates the surface in the shape of a wedge and the diverted ground configuration parallel to the axis presses against the material to be penetrated, with this ground portion acting like a plane and slicing off material which is pressed against it.
Another embodiment of a Huber cannula is known from U.S. Pat. No. 2,409,979. The embodiment described has a straight cannula tube which opens into a lumen opening directed toward the side, which is shaped such that side of the cannula wall opposite the lumen opening is bent at an angle in the direction of the axis. The wall surrounding the lumen opening is ground even with the outside surface of the cannula tube and forms a cutter on the rear edge.
Because of the beveled surface of the cannula tube, this embodiment as well exhibits a wedge effect during piercing of a synthetic or natural wall such that the material forced into the lumen opening during thrusting of the cannula, favored by the cutter which angles slightly inward, is sliced off or damaged by pinching.
Another cannula is known wherein there is a needle whose front piercing part is designed in the shape of a hook, whereby during the piercing of a vein, for example, the danger of injury to the opposing wall of the vessel is supposed to be prevented.
The front, rounded edge of the piercing part lies in the line of the outside wall of the cannula tube. It has no flat ground edge, while the rear edge of the lumen opening is ground flat to form a cutter. The side of the tube opposite the lumen opening forms a sharply curved surface which acts as a slip surface. This shape forces an angular incidence during piercing, with the course of the pierce following an insertion path corresponding to the radius of curvature of the slip surface. With this procedure, material of the surface to be penetrated is of necessity forced into the lumen opening, which material is cut as a plug by the flat rear cutter, and thus the pierce hole is likewise enlarged.
Finally, additional cannulae are known from GB-A-2 073 026 and U.S. Pat. No. 2,717,599 which, however, present the same problems as the two cannulae described.
In contrast, the object of the present invention is to design a cannula of a type, such that plug cutting is avoided during the piercing of natural or synthetic walls.